The present invention relates to a developing device for developing an electrostatic latent image formed on an image-forming surface and, more particularly, to a developing device for developing the electrostatic latent image by a magnetic developing agent.
In a developing process of an electrophotographic apparatus, a magnetic brush developing device using a single-component magnetic toner as a magnetic developing agent has been recently used.
A typical example is illustrated in FIG. 1. A developing roller 16 comprises a magnet roller 12 having a plurality of magnetic poles and a cylindrical sleeve 14 surrounding the magnet roller 12. The developing roller 16 is disposed opposing an electrostatic latent image-forming surface (surface to be developed) 10 of a photosensitive drum or image carrier, as shown in FIG. 1.
In the developing device of this type, a magnetic brush 20 of a magnetic toner 18 is formed on an outer surface of the cylindrical sleeve 14 by rotating the magnet roller 12 and the cylindrical sleeve 14 in opposite directions (clockwise and counterclockwise, respectively, as indicated by arrows in FIG. 1); rotating the magnet roller 12 and the cylindrical sleeve 14 in the same direction to form the magnetic brush 20 thereon, as shown in FIGS. 2 or 3; or rotating only the magnet roller 12. The magnetic brush 20 is controlled by a doctor blade 22 so as to have a uniform thickness and is brought into slidable contact with the electrostatic latent image-forming surface 10.
In a developing device of another type, as shown in FIG. 4, a stationary magnet roller 12' is used, while a cylindrical sleeve 14 is rotated to develop an electrostatic latent image.
These conventional developing devices can provide relatively good images. However, the following problems remain unsolved. In the device shown in FIG. 1, the most balanced image can be obtained. However, the magnetic toner 18 which is deposited on or is about to be deposited on the electrostatic latent image-forming surface 10 rotates upon rotation of the magnetic poles. For this reason, it is found that an image defect phenomenon called "trail" occurs at the trailing or leading end of a solid image portion S having a great amount of toner. The trail phenomenon appears like a comet tail at a trailing end of the solid image portion S of a copying paper sheet P in the direction of conveyance, as indicated by reference symbol T, in FIG. 5.
The mechanism of the trail phenomenon will be described with reference to FIG. 6 and FIGS. 7A to 7C. FIG. 6 shows rotation of the magnet roller 12 and corresponding movement of the magnetic toner 18. When the magnet roller 12 rotates in the direction indicated by arrow A, the magnetic toner 18 on the sleeve 14 moves in the direction indicated by arrow B. The magnetic toner 18 deposited on part of the electrostatic latent image-forming surface 10 opposing the sleeve 14 rotates by itself and moves in the direction indicated by arrow C (i.e., the toner itself rotates and moves in the same direction as the magnet roller 12).
The trail phenomenon occurs due to movement of the toner in the direction indicated by arrow C. The mechanism of the trail phenomenon is illustrated in FIGS. 7A to 7C. As shown in FIG. 7A, a toner chain 18' is cut off by an electrostatic attraction force of the electrostatic latent image-forming surface 10 and the magnetic force of the magnet roller 12. Several particles of the magnetic toner 18 which are located at the distal end of the cut toner chain are deposited on the electrostatic latent image-forming surface 10. These toner particles are moved in the same direction as the magnet roller 12 rotates (i.e., in the direction opposite to the rotational direction of the electrostatic latent image-forming surface 10), as shown in FIG. 7B. By this movement, as shown in FIG. 7C, part of the toner at the trailing end of the image is deposited at a prospective image-forming portion, thereby effecting the trail phenomenon. The trailed toner particles tend to roll to any other area outside the image-forming portion due to a low potential at this area.
The basic mechanism of the trail phenomenon has thus been described. Another cause of trailing is non-charged toner mixing. Non-charged toner particles tend to receive the influence of the magnet since the attraction force of the photosensitive drum is decreased. For this reason, trailing is typically observed immediately after the toner is replenished. When copying is repeated after the toner is replenished, trailing tends not to occur. Therefore, it is assumed that this phenomenon is partially caused by non-charged toner particles.